Re: JetPacks vs. AirCars

Robert J. Bradbury (
Sun, 22 Aug 1999 19:11:38 -0700 (PDT)

Ok, after being soundly jumped on by Billy, Doug, Phil, Spike, and probably a whole bunch of people who haven't even read this thread yet, *I RECANT". This will teach me to comment on things with incomplete knowledge without investigating a bit more (maybe :-)).

I was unaware of the effective ISP for airplanes/helicopters/etc. and am still not sure I fully understand the reasons for this. But I'll do some more reading and thinking on the subject.

The lack of a requirement for oxidizer in air-breathing rotors/turbines would appear to be a big part of the problem. I was thinking primarily of the thermodynamic efficiency and assumed that rockets had to be much better, apparently, not good enough to offset the additional weight of the oxidizer. I suspect that rockets will start to compete with the atomic fuel mentioned in the recent NASA news item [~4-5x higher than H2/O2 ISP (or perhaps 10x the JetPack's ISP) I believe].

So aircars are better because they don't require as much propellent.

Probably the ideas regarding rocket JetPacks (really Rocket-Packs) and JoSH's aircars (URL: have been having sex in my mind and been turned into Air-NanoturbinePacks. I believe that my original premise should have said that Air-NanoturbinePacks beat AirCars for the lower fuel-to-weight ratios. As others have pointed out there is a drag problem and you may need a nice pointy ShroudCoat. At that point I think you have an Air-Nanoturbine-Rocket.

The question becomes, can an Air-Nanoturbine-Rocket launch without oxidizer and turn itself into an Air-Nanoturbine-Ramjet-Rocket in the atmosphere on the way up (so you gather the oxidizer necessary for the final push to orbit above 40,000 ft)?

And does this get me personal ground-to-orbit transport for less energy cost than a cross-country trip?

And just for comparison purposes... Can I harvest the CO2 and H2O out of the atmosphere to build myself a nice plastic/diamond cable reinforced ballon and fill it with H2 and "float" part of the way up for an even cheaper ride? I.e. is the energy cost to harvest materials & construct the balloon cheaper than the energy cost of the fuel to get to the same height?

On Sun, 22 Aug 1999, Spike Jones wrote:

> This lack of understanding of the difficulty of a task leads to the
> comments I have heard since childhood: "If those smart scientists
> can put a man on the moon, you would think they could...[fill in
> the blank]" This blank is often filled with some comment that makes
> no sense at all, such as "build a car that runs on water" or equivalent.

But we can *build* a car that runs on water. Put the water in the tank, let the solar cells turn it into H2, run the H2 into a fuel cell and then drive off. We just can't build one at the same "cost" as current automobiles. This is in large part a problem of production volumes. If we could produce the fuel-cell cars in the same volumes as production model cars, the costs would be much lower. Historically there may have been problems with fuel cell materials (e.g. platinum), but these are either solved now or could be solved by recycling.

The problem isn't with the science, it is with the unflexible manufacturing infrastructure.